In order to obtain a good end-product, i.e. good cellulose pulp, when digesting lignocellulosic material, it is necessary to steam the chips well, prior to adding cooking chemicals thereto. In the continuous digestion of wood chips, it is normal practice to pass the chips through a steaming vessel to which steam is introduced, this steam comprising mainly withdrawn steam, i.e. steam which has been generated in and recovered from somewhere in the pulp manufacturing process. This steam is often fortified with fresh low-pressure steam, particularly during the winter time.
In the winter time, when the chips introduced into the steamer are often frozen, it is difficult to obtain complete penetration of the steam into and through all the chip portions. Steaming can also be unsuccessful to varying degrees in other parts of the year. This can result in serious difficulties when digesting wood to cellulose pulp. Incomplete steaming of the chips means that air remains therein. The air may remain in such quantities as to render the chips buoyant, so that they float in the cooking liquor. It has been found, that the chip column may cease to move down through the continuous digester altogether.
In recent years, it has therefore become more and more usual to pre-heat the chips, so as to facilitate and improve the steaming process. One method of pre-heating the chips is to feed them to a cylindrical vessel and to apply heat to the periphery thereof with the aid of a heating medium, normally steam of some kind. It has been found difficult to heat the whole of the chip flow. In particular, those chips located in the centre of the vessel are not always heated satisfactorily. If the flow of steam is increased in order also to heat the chips located in the centre of the vessel to a sufficient extent, steam will often penetrate through the chip column. Problems then occur in the apparatus located upstream of the pre-heating stage, for example chip screws or belt conveyors. Another disadvantage with pre-heating methods of the aforedescribed kind is that the condensate formed accompanies the chips and dilutes the cooking liquor. Furthermore turpentine is lost with this condensate.